Method for treating a silicon substrate, by nitriding, to form a thin insulating layer

ABSTRACT

Method for preparing a silicon substrate to form a thin electric insulating layer ( 24 ), characterized in that it comprises: 
     a deoxidation step of at least one part of the silicon substrate ( 10 ), then 
     a heat treatment step of the substrate at a temperature of 750° C. or less, the heat treatment being conducted in a NO-containing atmosphere at a pressure of 5.10 3  Pa (50 mBr) or less, in order to form a layer of silicon oxynitride ( 22 ) on the substrate. Use for the production of EPROM and DRAM memories.

“This application is a national phase of PCT/FR99/02228 which was filedon Sep. 20, 1999, and was not published in English.”

TECHNICAL FIELD

The present invention relates to a method for treating a siliconsubstrate for the purpose of forming, on at least one of its surfaces, alayer of electric insulating material such as, for example, a layer ofsilicon nitride.

The invention finds applications in the production of electronic deviceswith components having a thin electric insulating layer, and inparticular for the production of DRAM type memories (dynamic randomaccess memory) or EPROM memories (erasable/programmable read onlymemory).

It may also be applied to the production of electronic circuits havinginsulated gate transistors such as MOS transistors or other componentssuch as capacitors.

STATE OF THE PRIOR ART

The increased performance of electronic components in terms offrequency, integration and electric capacity for memories, isaccompanied by a reduction in the thickness of the electric insulatinglayers, in particular of the gate layers of these devices.

The gate layer, for components made on a silicon substrate is usually alayer of silicon oxide.

The reduction in the thickness of the oxide layer to values of less than3 nm gives rise to problems relating to the diffusion of dopingimpurities derived from overlying active layers, through the oxidelayer. This diffusion has adverse effects on the reliability andperformances of the components comprising the oxide layer.

The problem of the diffusion of doping impurities may be remedied, atleast in part, by incorporating in the gate layer oxide of thecomponents an appropriate dose of nitrogen, in particular by means of anitriding treatment. In particular, the oxide layer may be combined withor optionally replaced by a layer of silicon nitride.

Also, to illustrate the fabrication of thin nitride layers in DRAM andEPROM structures, reference may be made to documents [1], [2], [3], [4]and [5] whose references are specified at the end of this disclosure.

Document [1] in particular shows that it is not possible to form ahomogeneous, continuous nitride layer thinner than 5 nm on a layer ofnative oxide on the surface of a substrate.

For applications such as the fabrication of memories, however, gatethicknesses of less than 5 nm are required.

Document [2] suggests solving the problems of continuity ornon-homogeneity of the thin nitride layers (<3 nm) by subjecting them toquick annealing in an atmosphere of NH₃ at temperatures in the order of950° C. Nevertheless, it arises that such annealing, owing to its hightemperature, may deteriorate the electronic components previously formedin the substrate.

Documents [3] and [4] describe techniques with which a layer of nativeoxide, initially present on the surface of a silicon substrate, isremoved before the formation of a nitride layer by chemical vapourdeposition on the exposed silicon surface. Deoxidation of the substratemay take place by annealing under hydrogen or by chemical means usinghydrofluoric acid.

Finally, document [5] proposes forming on the substrate a layer ofsilicon oxynitride, prior to the layer of silicon nitride. Theoxynitride layer is formed in an atmosphere of NO. The silicon nitridelayer is then is formed from the gases SiH₄ and NH₃ in a reactor ofmonoplate type. Enriching the treatment gases with silane (SiH₄)promotes nucleation of the silicon nitride but deteriorates itsstoicheiometric quality. The use of a monoplate reactor is also scarcelycompatible with industrial production of components with low productioncosts.

The methods of documents [3], [4] and [5] also entail treatments at hightemperatures, in the order of 800°C. to 1000° C., and use high heatschedules.

For a certain number of components, however, in particular structures ofembedded DRAM type, it is sought on the contrary to reduce the heatschedules as much as possible, that is to say the time and length ofheat treatments. High heat schedules and high treatment temperatures areharmful for the components.

DISCLOSURE OF THE INVENTION

The object of the invention is to put forward a method for preparing asubstrate with which it is possible to form a thin layer of electricinsulator which does not have the above-mentioned difficulties.

One object in particular is to put forward such a method enabling theformation of a continuous, homogeneous, thin nitride layer on a siliconsubstrate.

A further object of the invention is to put forward a method which useslower heat schedules and temperatures.

To reach these objects, the subject of the invention is more precisely amethod for treating a silicon substrate so as to form a thin, electric,insulating layer. In accordance with the invention, the methodcomprises, in order:

a deoxidation step of at least part of the silicon substrate, then

a heat treatment step of the substrate at a temperature of 750° C. orless, the heat treatment being made in a NO-containing atmosphere, at apressure of 5.10³ Pa (50 mBar) or less, and preferably less than 10³ Pa(10 mBar) in order to form on the substrate a layer of siliconoxynitride, and

a formation step to form, at least on said part of the substrate, alayer of electric insulating material.

By a NO-containing atmosphere is meant an atmosphere of pure NO or NOdiluted with an inert gas such as nitrogen or argon.

With the heat treatment it is possible, on the surface of the deoxidisedpart of the substrate, to form a very fine layer of silicon oxynitridewhose thickness may be less than one nanometre. This layer enables thesubsequent formation of a thin insulating layer that is homogeneous andcontinuous.

Moreover, the oxynitride layer prevents the formation on the substrateof parasite deposits of metallic oxides such as Ta₂O₅ whose onset mayoccur during oxidizing treatments.

The heat treatment of the method is applied at temperatures of less than750° C., for example at a temperature in the order of 550° C. The methodmay therefore be applied to substrates comprising electronic componentsthat are relatively sensitive to heat, previously formed.

Preferably, the heat treatment may be applied for a sufficient length oftime to obtain an oxynitride layer having a thickness of between 0.5 and1.5 nm.

As an example, the heat treatment may be conducted at a temperature inthe region of 550° C., a pressure in the order of 10³ Pa (10 mBar) for atime of approximately 30 seconds to obtain an oxynitride layer of 0.7nm.

The silicon substrate used may have been previously subjected to priortreatment in order to form electronic components therein or parts ofelectronic components.

The layer of electric insulating material formed on the substrate may bea layer of silicon nitride (Si₃N₄) or a layer of Ta₂O₅ chosen for theirstrong dielectric constant.

In respect of a layer of silicon nitride Si₃N₄, this may be preferablyformed by a method of LPCVD type (Low Pressure Chemical VapourDeposition) in the presence of an atmosphere containing dichlorisilane(SiH₂Cl₂) and/or ammonia NH₃. The deposit is made at a temperature of750° C. or less, for example 700° C.

The invention also concerns a substrate, which may be obtained accordingto the above-described method and which, in order, comprises a layer ofsilicon with at least one area devoid of native oxygen, a layer ofsilicon oxynitride having a thickness of between 0.5 and 1.5 nm incontact with said area, and a layer in an electric insulating materialhaving a thickness of between 2 and 5 nm in contact with said layer ofsilicon oxynitride. The electric insulating material may be chosen fromamong Si₃N₄ and Ta₂O₅ for example.

Other characteristics and advantages of the invention will become betterapparent from the following description with reference to the figures ofthe appended drawings. This description is given solely for illustrationpurposes and is not restrictive.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of one portion of a silicon substrate, beforeapplying the preparation method according to the invention.

FIGS. 2 and 3 are successive diagrams of the portion of substrate shownin FIG. 1 after the deoxidation and heat treatment steps of theinvention.

FIG. 4 is a diagram of a portion of the substrate shown in FIG. 3 onwhich a thin insulating layer has been formed.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS OF THE INVENTION

FIG. 1 shows one part of a silicon substrate 10, monocrystalline orpolycrystalline, with a free surface designated by the reference 12.

Surface 12 is coated, before the treatment, with an oxide layer 14. Theoxide layer 14 may be a layer of native oxide which is naturally formedby contact of the silicon with air, or an oxide layer obtained by a heattreatment.

The silicon substrate may contain components or parts of components,such as transistor channels or memory structures for example. Thesecomponents or parts of components shall not be described in detail here,nor are they shown in the figures since they may vary depending upon theconsidered application.

A first step of the method is a deoxidation step whose purpose is toremove the oxide layer 14.

Deoxidation may be made by chemical means by immersing the substrate 10in a solution of hydrofluoric acid diluted in water. The concentrationof the acid is in the order of 1%, even lower.

After the first step a substrate according to FIG. 2 is obtained, forwhich the free surface 12 is exposed.

As shown in FIG. 3, the substrate is then placed in a chamber 20 inwhich an atmosphere of NO is set up.

The gas pressure in the chamber 20 is in the order of 5.10³ Pa (50mBar), or less.

In this chamber, the substrate undergoes a heat treatment at atemperature of less than 750° C., and preferably less than 700° C. whenthe fabricated components are DRAMs, to form a layer 22 of siliconoxynitride, having the formula SixNyOz, on surface 12. (The parametersx, y and z are stoicheiometric parameters).

Table I below gives the proportions of Si, O and N of the oxynitridelayer 22 for heat treatments conducted at 550° C. and 700° C., at apressure of 10³ Pa and for 30 seconds. The table also gives thethickness of the layers of silicon oxynitride obtained.

TABLE I Composition Si % O % N % Thickness 700° C. 35 49 16 0.92 nm 550°C. 37 45 17 0.65 nm

Table I shows that the composition of the layer of silicon oxynitrideundergoes little change with treatment temperature. The thickness,however, is affected.

The substrate so prepared may receive an electric insulating layer. Inthe described example, and as shown in FIG. 4, a layer of siliconnitride 24 is formed on the oxynitride layer 22.

The formation of the silicon nitride may take place in an oven 30 inwhich an atmosphere containing a mixture of NH₃/DCS(ammonia/dichlorosilane) is set up.

The formation of the nitride takes place by low pressure chemical vapourdeposition (LPCVD) at a temperature of less than 750° C., for examplebetween 700° C. and 750° C.

The nucleation properties of the silicon nitride on substrate 10 arelargely improved through the presence of the layer of silicon oxynitride22 which overcomes delay in nucleation. By nucleation properties ismeant in particular the kinetic properties of nucleation comprising theincubation time/ and or the density of the nucleation sites formed aftera certain time period.

If Ta₂O₅ is used as insulator, the oxynitride layer prevents oxidationbetween Si and Ta₂O₅.

By way of illustration, table II gives the thickness of the layers 22 ofsilicon oxynitride and of layers 24 of silicon nitride for three samplestreated differently.

A first control sample did not undergo the method of the invention, butcomprised a layer of silicon oxide on its surface. Two other sampleswere prepared in accordance with the invention in an atmosphere of NO at10³ Pa for 30 seconds. The samples were then given a LPCVD deposit ofsilicon nitride under equivalent conditions, at 700° C., with a NH₃/DCSratio of 9, and for a time period in the order of 10 to 20 minutes.

TABLE II Thickness Thickness Nitriding of of conditions/ oxynitridenitride NO layer 22 layer 24 Difference Sample 1 none 0.9 nm 3.5 nm 2.55nm (control) (oxide layer) Sample 2 550° C./30″ 0.7 nm 4.44 nm  3.74 nmSample 3 700° C./30″ 0.92 nm  4.5 nm 3.58 nm

Table II shows a change in nucleation delay. The existence of the layerof silicon oxynitride 22, under identical LPCVD deposit conditions,makes it possible to obtain more rapid nitride formation.

Also, the nitride layers 24 are homogeneous and continuous, despitetheir narrow thickness.

Cited Documents

[1] FR-98 01963

[2] L. F. Tz Kwakman, E. J. Lindow, E. H. A. Granneman, F. Martin, J. C.Veler and J. P. Joly, Applied Surface Science 70/71, p. 629-633 (1933).

[3] S. Saida, T. Sato, I. Mizushima, Y. Ozawa, Y. Tsunashima, ExtendedAbstract of the IEDM, p. 265 (1997).

[4] K. Kobayashi, Y. Inaba, T. Ogata, T. Katayama, H. Watanabe, Y.Matsui, M. Hiramaya, Journal of the Electrochemical Society, vol. 143,No. 4, p. 1459 (1996).

[5] B. Y. Kim, H. F. Luan, D. L. Kwong, Extended Abstract of the IEDM,p. 463 (1997).

[6] F. Martin, F. Bertin, H. Sprey, E. Granneman, Semicond. Sc. Technol.6, p. 1100 (1991).

What is claimed is:
 1. A method for treating a silicon substratecomprising: a deoxidation step of at least one part of the siliconsubstrate (10); then a heat treatment step of the substrate at atemperature of 750° C. or less, the heat treatment being conducted in aNO-containing atmosphere at a pressure of 5.1·10³ Pa (50 mBar) or less,wherein said heat treatment step is applied for a sufficient length oftime to obtain a layer of silicon oxynitride (22) having a thickness ofbetween 0.5 and 1.5 nm, and a formation step wherein a layer of electricinsulating material is formed on the layer of silicon oxynitride. 2.Method according to claim 1, in which deoxidation is conducted bychemical means and by immersing the substrate in a dilute solution ofhydrofluoric acid.
 3. Method according to claim 1 in which the heattreatment is conducted at a temperature 550° C., a pressure in the orderof 10³ Pa (10 mBar) for a time period of approximately 30 seconds. 4.Method according to claim 1, in which the layer of electric insulatingmaterial (24) is formed by low pressure chemical vapour deposition. 5.Method according to claim 1, in which the layer of electric insulatingmaterial (24) is formed at a temperature of 750° C. or less.
 6. Methodaccording to claim 1, in which the electric insulating material isselected from the group consisting of Si₃N₄ and Ta₂O₅.
 7. Methodaccording to claim 1, in which the layer of electric insulating material(24) has a thickness of between 2 and 5 nm.
 8. Method according to claim1, in which the layer of electric insulating material is Si₃N₄ isformed, by low pressure chemical vapour deposition, in the presence ofSi₂H₂Cl₂.
 9. Substrate comprising, in order, a layer of silicon (10)with at least one area (12) devoid of native oxygen, a layer (22) ofsilicon oxynitride having a thickness of between 0.5 and 1.5 nm incontact with said area (12), and a layer of an electric insulatingmaterial having a thickness of between 2 and 5 nm, in contact with saidlayer of silicon oxynitride.
 10. Substrate according to claim 9, inwhich the electric insulating material is selected from the groupconsisting of Si₃N₄ and Ta₂O5.